Seismic waves generated artificially for the imaging of geological layers has been used for more than 50 years. The most widely used waves are by far reflected waves and more precisely reflected compressional waves. During seismic prospection operations, vibrator equipment (also known more generically as a “source”) generates a seismic signal that propagates in particular in the form of a wave that is reflected on interfaces of geological layers. These waves are received by geophones, hydrophones, and/or other types of receivers, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal recorded by means of recording equipment. Analysis of the arrival times and amplitudes of these waves makes it possible to construct a representation of the geological layers on which the waves are reflected.
FIG. 1 depicts a seismic exploration system (system) 70 for transmitting and receiving seismic waves intended for seismic exploration in a land environment. At least one purpose of system 70 is to determine the absence, or presence of hydrocarbon deposits 44, or at least the probability of the absence or presence of hydrocarbon deposits 44. System 70 comprises a source consisting of a vibrator 71 operable to generate a seismic signal (transmitted waves), a plurality of receivers 72 (or geophones) for receiving seismic signals and converting them into electrical signals, and seismic data acquisition system 200 (that can be located in, for example, vehicle/truck 73) for recording the electrical signals generated by receivers 72. Source 71, receivers 72, and data acquisition system 200, can be positioned on the surface of ground 75, and interconnected by one or more cables 12. FIG. 1 further depicts a single vibrator 71, but it should be understood that source 71 can actually be composed of multiple or a plurality of sources 71, as is well known to persons skilled in the art.
In operation, source 71 is operated so as to generate a seismic signal. This signal propagates firstly on the surface of ground 75, in the form of surface waves 74, and secondly in the subsoil, in the form of transmitted ground waves 76 that generate reflected waves 78 when they reach an interface 77 between two geological layers. Each receiver 72 receives both surface wave 74 and reflected wave 76 and converts them into an electrical signal in which are superimposed the component corresponding to reflected wave 78 and the one that corresponds to surface wave 74, the latter of which is undesirable and should be filtered out as much as is practically possible.
An example of a vibratory source 71 is shown in FIG. 2. Source 71 can include base plate 88 that connects to rod 80. Rod 80 includes piston 82 inside reaction mass 84. Insulation devices 86 can be provided on base plate 88 to transmit weight 90 of vehicle 73 to base plate 88. Base plate 88 is shown in FIG. 2 as lying on ground 75. The force transmitted to ground 75 is equal to the mass of base plate 88 times its acceleration, plus the weight of reaction mass 84 times its acceleration. The weight of vehicle 73 (shown in FIG. 1) prevents base plate 88 from losing contact with ground 75. Many designs for vibratory sources 71 exist on the market, and any one of them can be used with the embodiments discussed herein.
The signals recorded by seismic receivers 72 vary in time, having energy peaks that may correspond to reflectors between layers. In reality, since many other layers 77, and especially the weather layer, or low-velocity-layer (LVL) 79 (shown in FIG. 3) can be moderately to highly reflective, some of the peaks correspond to multiple reflections or spurious reflections that should be eliminated before the geophysical structure can be correctly imaged. Primary waves 78 suffer only one reflection from an interface between layers of the subsurface (e.g., first reflected signal 78a). Waves other than primary waves are known as multiples, and more strictly, are events that have undergone more than one reflection. Typically, internal multiples, which occur when energy is reflected downward at an interface layer in the subsurface, have a much smaller amplitude than primary reflected waves, because for each reflection, the amplitude decreases proportionally to the product of the reflection coefficients of the different reflectors (usually layers or some sort). As shown in FIG. 3, discussed below, there are several ways for multiples to be generated.
For example, consider that seismic source 71 produces transmitted waves 76a-d that become two primary reflected waves, and two multiple waves. Primary transmitted wave 76b penetrates through both first and second subsurface layers 16a,b (referred to also as, e.g., the “sediment layer”), and becomes first reflected signal 78a that reaches first receiver 72a. Primary transmitted wave 76d becomes second reflected signal 78b, and also reaches receiver 72a. Each of reflected signals 78a,b represent true information about the underlying GAI and it is one of the primary goals of seismic signal processing to find and enhance these types of signals, and eliminate the influence, to the greatest extent possible, of multiples 51, as further discussed below. Primary transmitted wave 76a reflects off first interface layer 77a and then off weather layer 79, down again to layer 77a, and then up to surface 75. This type of multiple is known as a “peg-leg” multiple 51b. In FIG. 3, peg-leg multiple 51b is not shown as having been received by receiver 72a (in fulfillment of the dual purposes of clarity and brevity), though as one of skill in the art can appreciate it generally will be, becoming “noise” that should be filtered out to the greatest extent possible. Primary transmitted wave 76c reflects off second interface level 77b, bounces up and down again off first interface level 77a, then up off second interface level 77b to reach receiver 72a as simple multiple 51a. Internal multiple signals 51a,b, and primary reflected signal 78a,b all reach receiver 72a, but at different times. Thus, receiver 72b can receive at least several different signals from the same transmitting event.
Attempts have been made to correct for the deleterious effects of multiples on seismic data by using internal multiples predictions. For example, U.S. Published Patent Application No. 2102/0253758, entitled “Method of Wavelet Estimation and Multiple Prediction In Full Wavefield Inversion,” describes a wavelet estimation method, allegedly advantageous for full wavefield inversion (FWI) of seismic data, which makes use of both the primary and multiple reflections in the data. The method uses an FWI algorithm to generate a subsurface model from primary reflections in a shallow layer before first arrival of multiple reflections. The model is then used to simulate multiples. The wavelet is subsequently modified such that the simulated multiples closely match the true recorded multiples. The simulated multiples may then be subtracted from the measured data thereby creating a deeper top layer of data substantially free of multiples, and the method may then be repeated to extend the subsurface model to a greater depth.
In addition, there is U.S. Published Patent Application No. 2011/0317521, entitled “Correcting Geometry Related Time and Amplitude Errors,” which discloses a method for predicting a plurality of surface multiples for a plurality of target traces in a record of seismic data acquired in a survey area. The method includes selecting a target trace and identifying two or more desired traces for multiple prediction based on the target trace. After identifying the desired traces, the method identifies one or more recorded traces for each desired trace. Each identified recorded trace is described as being substantially close to one of the desired traces. The method then includes correcting the identified recorded traces for one or more geometry-related effects associated with the survey area and convolving the corrected recorded traces to generate a plurality of convolutions. After convolving the corrected recorded traces, the method then stacks the convolutions.
Such techniques do not, however, effectively correct for all of the anomalies found in recorded seismic data. For example, such techniques do not adequately account for some near surface statics, including especially travel-time anomalies in the weathered zone, which are very difficult to obtain and correct for by direct measurements. Near surface statics, as those of skill in the art can appreciate, relate to those effects associated with weathering, discussed below, and elevation. As those of skill in the art can appreciate, the weather zone, or low-velocity-layer, is a zone, usually between about 4 to 50 meters thick, which is characterized by not only low seismic velocities, but also velocities that can be highly variable.
There are several significant effects of the weathered zone in the context of the quality of the recorded seismic data. First, the absorption of seismic energy in the zone can be significant; second, the low velocity and changes in velocity can have a disproportionately significant effect on travel times; third, because of the low velocity, the wavelengths are shortened and therefore smaller features produce significant scattering and other noise; fourth the marked bend in velocity at the base of the weather zone sharply bends the seismic signal such that their travel is almost always nearly vertical through the zone, regardless of travel below the weather zone; and fifth, the very high impedance contrast at the base makes it a very good reflector of seismic energy.
Accordingly, it would be desirable to provide methods and systems for correcting for the effects of near surface statics, including travel-time anomalies, on acquired seismic data by using internal multiples prediction.